1. Field of the Invention
The present invention relates to a magnetic barrel tumbler for polishing precious metal castings by utilizing magnetic force, and more particularly to a magnetic barrel tumbler for hardening surface structures of metal castings and polishing surfaces of the metal castings by forcing the metal castings to be struck against polishing metal pins in a polishing barrel by utilizing variations in the magnetic field generated by the rotation of a magnetic disc disposed beneath the polishing barrel.
2. Description of the Prior Art
Generally, precious metal articles having particular shapes such as finger rings are cast by molds. Since such precious metal castings have a slightly instable surface structure after molding, they are subsequently polished manually.
For articles made of a metal, e.g., gold, silver, platinum, titanium or nickel, or a hard plastic material exhibiting a high degree of hardness, it is difficult to manually harden the surface structure. In this case, it takes much time to obtain a polished surface.
In order to solve this problem, various polishing devices have been developed. For example, U.S. Pat. No. 5,044,128 discloses a magnetically-polishing machine for mechanically polishing articles by utilizing variations in the magnetic field occurring during the rotation of a magnetic disc attached with magnets.
The polishing machine of the U.S. patent employs a magnetic disc, the surface of which is divided into alternating south-polar sectors or zones and north-polar sectors or zones. As the magnetic disc is rotated at high speed by a motor, a magnetic force is induced. That is, variations in the magnetic field occur. Due to such variations in the magnetic field, polishing pins, which are small metal pins disposed in a polishing vessel on the magnetic disc, are vibrated to polish articles contained in the polishing vessel.
The magnetic polarity on the upper surface of magnetic disc which induces variations in the magnetic field at the bottom of polishing vessel is made such that the zones having the same polarity are arranged at opposite positions, respectively. In other words, the polarity arrangement is made in the order of N, S, N, S, . . . in one direction along the magnetic disc surface (FIG. 3B). In this patent, the matrix of the magnetic disc is made of a nonmagnetic material.
Above the magnetic disc, a fixed plate comprised of a metal plate is disposed, on which the polishing vessel is laid.
In this construction, however, a lot of heat is generated between the fixed plate and the magnetic disc as variations in the magnetic field occur during a rotation of the magnetic disc. The heat interferes with an increase in the magnetic field variation. Furthermore, the alternating current (AC) motor disposed beneath the magnetic disc is affected by the heat, thereby giving an adverse effect on the overall operation of the machine.
Such phenomenons result from the fact that the fixed plate disposed above the magnetic disc is made of a metal exhibiting a certain electrical resistance and that the matrix of the magnetic disc is made of a nonmagnetic material. Since the magnetic disc and the fixed plate are constituted by non-metallic and metallic plates, respectively, heat is easily generated during a high-speed rotation of the magnetic disc. Furthermore, the variable magnetic field induced by the high-speed rotation of the magnetic disc serves to generate a large quantity of heat when it passes through the metallic fixed plate.
In accordance with the patent, variations in the magnetic force are limited within a certain range so that the polishing work can be carried out only in the range. Due to such a limitation, it may be impossible to appropriately adjust the magnetic field variation range in accordance with the amount of articles to be polished in the polishing vessel. Where the amount of articles to be polished is large, appropriate polishing may not be achieved due to weak magnetic field. Since the bottom surface of polishing vessel is flat throughout all portions thereof, the central portion thereof is partially or not subjected to magnetic force. As a result, there is a problem that the efficiency of polishing work is degraded.
Meanwhile, barreling carried out in mechanical polishing machines is a process which mechanically agitates articles to be polished along with abrasives comprising polishing pins, polishing oil, etc. When a part of the polishing pins enter recesses defined in the articles being polished, they cannot move freely. As a result, efficient barreling cannot be achieved. In this regard, the vessel should have a shape capable of providing a smooth agitation of polishing pins. A strong force is required to provide a free agitation of polishing pins.